This invention relates to methods and systems for generating a broadband spectral continuum, methods of making the systems and pulse-generating systems utilizing same.
The bandwidth demands projected for the near future will require multi-Tb/s TDM/WDM systems that are based on numerous high bit-rate channels. A supercontinuum (SC) generated in optical fiber is a convenient source for such systems because it provides a very broad bandwidth ( greater than 200 nm) that can be sliced, as required, into short pulses at individual WDM channels as illustrated in FIG. 1. The pulse trains in each channel have the repetition rate of the source laser and, when the spectrum is flat and of uniform phase, pulse widths that are transforms of the spectral filter function. These features make the continuum source an attractive alternative to numerous discrete laser diodes, particularly for high bit-rate OTDM systems, since a single short pulse source provides chirp-free, ultra-short pulses simultaneously for multiple wavelength channels. Also, the SC source requires the relatively simpler stabilization of passive filters rather than of the operating wavelengths of multiple laser diodes. Finally, since the SC has a continuous high power spectral density outside the erbium gain band compared with thermal sources or superluminescent LEDs, it is also useful for characterizing passive components and amplifiers in new spectral regions.
Work in the 1980""s first explored continuum generation in fibers. Working in the anomalous group-velocity dispersion (GVD) regime of fibers, several groups generated short pulses with extremely broad bandwidth, which they attribute to stimulated Raman scattering. Using 100-psec pulses from a Nd:YAG laser, Gouveia-Neto et al. obtained a spectrum between 1.32 and 1.54 xcexcm.
By frequency doubling a 2.79 xcexcm YSGG:Cr3+:Er3+ laser, Vodop""yanov et al. generated 100-200 fsec pulses between 1.5 and 1.7 xcexcm.
On the other hand, Beaud et al. witnessed pulse breakup when 0.83-psec pulses from a 1.37 xcexcm dye laser were passed through a fiber.
Blow et al. tried to reconcile the difference between broad bandwidth and pulse breakup by theorizing that the frequency shift can be suppressed by Raman gain.
Islam et al. developed a model of the femtosecond distributed soliton spectrum (FDSS) that does not rely on stimulated Raman scattering at its peak (xcx9c440 cmxe2x88x921 below the pump frequency) but did explain all three of the above experiments. In particular, Islam et al. generated pulses with duration larger than xcx9c100-fsec between 1.55 and 1.85 xcexcm in a fiber pumped by a color center laser. The experiments and computer simulations showed that in the anomalous GVD regime, the narrow pulses evolve from multi-soliton collisions initiated by modulational instability and soliton self-frequency shift effects. These experiments were conducted in fiber lengths of 100 to 500 m of single mode, polarization-maintaining fiber. Cross-correlation measurements suggest that there is little or no correlation between spectral components of the FDSS that are separated by more than the 100-fsec pulse bandwidth. The experiments support a model of the FDSS as an ensemble average over fundamental solitons that have frequency shifted by different amounts.
More recently, Morioka et al. studied 1 Tb/s (100 Gb/s xc3x9710 channels) TDM/WDM transmission using a single SC WDM source. In particular, and as illustrated in FIG. 2, their SC is generated in 3 km of fiber, and their SC source has a bandwidth  greater than 200 nm. Also, they used dispersion decreasing fiber with the third order dispersion flattened. Their pulses are compressed using adiabatic soliton compression (ASC) and spectral shaping is achieved through normal GVD propagation.
The most striking feature of the Morioka et al. SC is that it can generate short pulses  less than 0.3 ps over the continuous spectral range, and that multi-wavelength, transform-limited short pulses can easily be selected by filtering with passive optical filters as illustrated in FIG. 1. The optical frequency stability of the filtered channels was quite high (xcx9c1 GHz/C), determined by that of the filtering devices. Morioka demonstrated 100 Gb/sxc3x9710 channel optical signal generation and error-free transmission of all the 100 Gb/sxc3x9710 channels over 40 km of DS fiber using the low-noise SC WDM source with a newly developed array-waveguide grating demultiplexer and multiplexer. ASC derives from a fundamental N=1 soliton""s tendency to decrease its pulse width to maintain a constant area in response to gradually decreasing dispersion or increasing energy with propagation. When resulting from a dispersion-decreasing fiber the amount of compression depends on the ratio of the initial to final dispersions. In addition, by using a fiber in which the 3rd order dispersion is flattened near the center wavelength and symmetrically concave about it, the spectrum broadens symmetrically. Also, in fiber with flattened 3rd order dispersion, stimulated Raman scattering is the dominant higher-order mechanism that shapes the continuum. Therefore, for continuum generation based on ASC in long fibers, optimization generally requires:
operation over the fiber length in both the normal and anomalous dispersion regimes;
specially designed dispersion fiber; and
suppression of the 3rd order dispersion.
A group at the University of Michigan has further optimized the SC generation in long fibers by using dispersion decreasing (DD) fibers. Using DD fiber can enhance the SC generation process. For example, using 3.3 km of DD fiber with 24.3W peak input power, one can obtain 100 nm SC that is flat over more than 20 nm and twice as broad and more uniform spectrum than dispersion-increasing or constant dispersion fiber. The DD fiber generates a broader and smoother spectrum than the other fibers because the changing zero dispersion wavelength enhances the generation of new frequencies, as self-phase modulation effects are more efficient near the zero dispersion wavelength.
Following the University of Michigan group""s work on dispersion tailored fibers for continuum generation, a number of groups have studied optimization of the continuum in kilometer lengths of specialty dispersion fibers. K. Mori et al. show that the SC spectrum can be optimized by using DD fiber in which the dispersion is a convex function of frequency with two zero-dispersion wavelengths.
Okuno et al. show experimentally the generation of 280 nm wide continuum by using a kilometer length of dispersion-flattened and decreasing fiber. As illustrated in FIG. 5, pulses are compressed by adiabatic soliton compression. Spectral shaping is accomplished through normal GVD propagation.
In contrast, Sotobayashi and Kitayama demonstrate 325 nm wide continuum by using a two state set-up: the first stage for pulse compression and the second stage for continuum generation. As illustrated in FIG. 4, pulses are compressed by soliton-effect compression and spectrum shaping is accomplished through normal GVD propagation. The pulses are first compressed in a 4 km length of fiber with anomalous dispersion through the higher-order soliton compression effect. Then, continuum is generated in a 2 km length of dispersion-flattened fiber that has a constant normal dispersion throughout the fiber length.
In a similar fashion, Takushima et al. generate over 140 nm-wide supercontinuum from a normal dispersion fiber by using a mode-locked semiconductor laser source. In their two-stage set-up, the pulses are first compressed through the adiabatic soliton compression technique in a 10.2 km length of DD fiber as illustrated in FIG. 3. An EDFA is used to boost the signal after the compression, and the output is then fed into a 1.7 km length of dispersion-flattened fiber with normal dispersion to generate the continuum through normal GVD propagation.
All of the experiments illustrated in FIGS. 2-5 however require some type of non-commercial, specialty fiber.
Although continuum sources have been demonstrated as described above, the problem has been that fiber lengths of typically 3 km have been required. This long fiber length leads to several problems: (a) the timing of pulses at different wavelengths is different, depending on where the spectrum is generated in the fiber; (b) the mechanisms are difficult to isolate since the long fiber is highly nonlinear; (c) the continuum is sensitive to polarization and environmental fluctuations; and (d) timing jitter results from the long interaction length with amplified spontaneous emission and dispersive waves.
An object of the present invention is to provide a method and system for generating a broadband spectral continuum, a method of making the system and pulse-generating system utilizing same wherein the continuum is generated in short lengths of optical fiber that can provide multiple wavelengths with single channel repetition rates of 1-100 Gb/s or higher.
Another object of the present invention is to provide a method and system for generating a broadband spectral continuum, a method of making the system and a pulse-generating system utilizing same wherein the continuum is generated using a multi-stage, soliton-effect compression.
Still another object of the present invention is to provide a method and system for generating a broadband spectral continuum, a method of making the system and a pulse-generating system utilizing same wherein the continuum is generated almost exclusively by self-phase modulation (SPM) and is shaped primarily by second and third order dispersion effects.
In carrying out the above objects of the present invention, a method for generating a broadband spectral continuum from a higher-order soliton pulse is provided. The method includes the steps of compressing the higher-order soliton pulse in a temporal domain through soliton-effect compression to obtain a compressed soliton pulse having a spectrum and breaking up the compressed soliton pulse to shape the spectrum of the compressed soliton pulse through higher order dispersion effects and self-phase modulation to obtain the broadband spectral continuum.
Preferably, the step of compressing includes the step of launching the higher-order soliton pulse into a first end of an anomalous dispersion fiber including at least one pulse compression stage having a length based on the order of the soliton pulse and a spectral shaping stage.
Also, preferably, the step of breaking up is performed adjacent a second end of the anomalous dispersion fiber in the spectral shaping stage.
The spectral shaping stage may be a dispersion-shifted optical fiber section of the anomalous dispersion fiber and the step of compressing may be performed in multiple pulse compression stages of the anomalous dispersion fiber.
Preferably, the higher order dispersion effects include third order dispersion effects and the shape of the broadband spectral continuum is based primarily on the sign and magnitude of third order dispersion of the spectral shaping stage adjacent the second end.
The shape of the broadband spectral continuum is also based on pulse width of the compressed soliton pulse immediately prior to the step of breaking up. The dispersion effects include second and third order dispersion effects. The magnitude of the second order dispersion and the magnitude of the third order dispersion normalized by the pulse width have substantially the same order of magnitude adjacent the second end of the anomalous dispersion fiber in the spectral shaping stage. Preferably, the spectral continuum is greater than 10 nm wide.
Further in carrying out the above objects and other objects of the present invention, a system for generating broadband spectral continuum from a higher-order soliton pulse is provided. The system includes at least one pulse compression stage of anomalous dispersion fiber having a length based on the order of the soliton pulse for compressing the soliton pulse through soliton-effect compression to obtain a compressed soliton pulse having a spectrum. The system also includes a spectral shaping stage of the anomalous dispersion fiber optically coupled to the at least one pulse compressor stage of anomalous dispersion fiber for breaking up the compressed soliton pulse adjacent an output end of the fiber to shape the spectrum of the compressed soliton pulse through higher order dispersion effects and self-phase modulation to obtain the broadband spectral continuum.
Preferably, the at least one pulse compression stage of anomalous dispersion fiber has a dispersion which is relatively constant therein.
Also, preferably, the anomalous dispersion fiber has a second pulse compression stage less than 100 meters in length and may be less than 10 meters in length.
The dispersion effects include third order dispersions having a sign and a magnitude. The sign may be positive or negative.
The system may have a plurality of pulse compression stages of anomalous dispersion fiber for compressing the soliton pulse through soliton-effect compression to obtain the compressed soliton pulse. Preferably, the length of the at least one pulse compression stage of anomalous dispersion fiber is based on a minimal width of the compressed soliton pulse.
Still further in carrying out the above objects and other objects of the present invention, a system for generating pulses substantially simultaneously on multiple channels at multiple wavelengths and at repetition rates of at least 1 Gb/s per wavelength channel is provided. The system includes a soliton pulse generator including a single laser for generating soliton pulses and an optical fiber including at least one pulse compression stage and a spectral shaping stage. The at least one pulse compression stage receives the soliton pulses at a first end of the fiber. The spectral shaping stage generates a broadband spectral continuum within the fiber and provides the broadband spectral continuum at a second end of the fiber. The system also includes a plurality of filters coupled to the second end of the fiber for carving broadband coherent outputs from the broadband spectral continuum to obtain pulses having multiple wavelengths and repetition rates of at least 1 Gb/s per wavelength channel.
The single laser may be a single mode-locked laser such as a mode-locked erbium-doped fiber laser.
The plurality of filters may comprise a plurality of passive filters. The passive filters set wavelength stability and the pulses"" spectral and temporal widths.
Preferably, the length of the spectral shaping stage is less than 10 meters and may be a dispersion-shifted fiber.
The optical fiber may be a polarization preserving fiber.
The optical fiber may be a high-nonlinearity fiber, in which       γ     greater than           2.2      ⁢              xe2x80x83            ⁢              km                  -          1                    ⁢              W                  -          1                      ;      γ    =                  2        ⁢                  π          ·                      n            2                                      λ        ⁢                  (                      A            eff                    )                    
The soliton pulse generator may also include a polarization controller and a fiber amplifier such as an erbium-doped fiber amplifier.
Yet still further in carrying out the above objects and other objects of the present invention, a method is provided of making an anomalous dispersion fiber including at least one pulse compression stage for generating a compressed soliton pulse having incident pulse energy, Po, and pulse width, xcfx84, from a higher-order, N, soliton pulse also having incident pulse energy, Po, and pulse width, xcfx84. The fiber also includes a spectral shaping stage for generating a broadband spectral continuum from the compressed soliton pulse. The method includes the steps of determining compression parameters including dispersion, D, and length, L, for the at least one pulse compression stage based on Po and xcfx84 of the higher-order soliton pulse. The method also includes the step of determining shaping parameters including D, L, and dD/dxcex for the spectral shaping stage based on xcfx84, Po and chirp, C, of the compressed soliton pulse. Still further, the method includes the steps of providing the at least one pulse compression stage having the compression parameters and the spectral shaping stage having the shaping parameters. Finally, the method includes the step of optically coupling the at least one pulse compression stage and the spectral shaping stage together to obtain the anomalous dispersion fiber.
Preferably, 300 fsecxe2x89xa6xcfx84 (of the higher-order soliton pulse)xe2x89xa63 psec.
Also, preferably Po (of the higher-order soliton pulse)xcx9c1.5xe2x89xa6Nxe2x89xa64 where:   N  =            [                        2          ⁢          πcγ          ⁢                      xe2x80x83                    ⁢                                    P              0                        ⁡                          (                              τ                1.763                            )                                                            λ            2                    |          D          |                    ]              1      /      2      
Preferably, D (of the at least one pulse compression stage) is xe2x89xa72 ps/nm-km.
Also, preferably, L (of the at least one pulse compression stage) is 2-100 m.
Preferably, xcfx84 (of the compressed soliton pulse) xe2x89xa6300 fsec.
Also, preferably, Po (of the compressed soliton pulse)xcx9c1.5xe2x89xa6Nxe2x89xa64.
Preferably, D (of the spectral shaping stage) is 0.2-2 ps/nm-km.
Also, preferably, dD/dxcex is xc2x10.01-0.07 ps/nm2-km.
Preferably, L of the spectral shaping stage is 0.3-100 m.
Also, preferably, C is linear and xe2x88x921xe2x89xa6Cxe2x89xa61 where C=(xcex94xcfx89xcex94xcfx84)2xe2x88x921.
In contrast to the prior art, the method and system of the present invention require only several meters of fiber. In preliminary experiments, 2.5 m of standard fiber followed by 2 m of dispersion-shifted (DS) fiber generate more than 200 nm of spectral continuum that is flat to less than xc2x10.5 dB over 60 nm as illustrated in FIG. 7. The continuum exhibits excellent piecewise coherence as evidenced by obtaining  less than 500 fs pulses that are pedestal-free to  greater than 28 dB, even when the spectrum is carved more than 70 nm from the pump wavelength. Also, the timing jitter of the carved pulses indicates no degradation compared to the source laser. Experiments match simulations well and indicate that the continuum is generated almost exclusively by self-phase modulation (SPM) and is shaped primarily by 3rd order dispersion effects.
By using multi-stage soliton compression in this approach, the continuum is generated in conventional fibers that are almost three orders-of-magnitude shorter in length than those used in related experiments, which are based on adiabatic soliton compression in specialty, dispersion-tailored fibers. As a consequence, a very stable source is obtained that has potential applications in high-capacity TDM/WDM applications. The short fiber continuum has:
multi-wavelengths that are generated coincident in time;
well-understood and controllable mechanisms underlying the continuum generation;
minimal interaction between the pulse and dispersive or incoherent energy;
high spectral coherence and low timing jitter; and
stability against mechanical and environmental fluctuations.
As compared with the prior art on supercontinuum generation in kilometer lengths of fiber, the present invention is novel because it relies on a different soliton compression mechanism and can be achieved in commercially available fibers. Whereas the continuum generation in kilometer lengths of fiber relies typically on adiabatic soliton compression (ASC), continuum generation in meter lengths of fiber relies on soliton-effect compression (SEC). Pulse compression is a key mechanism to continuum generation because the ideal continuum is the transform of a pulse of negligible temporal extent.
SEC typically requires launching of higher-order (Nxe2x89xa71.5) solitons, which periodically compress and expand with propagation in the anomalous dispersion regime of a fiber. Depending on the soliton order, the pulse achieves a minimal width at some fraction of the soliton period. When the fiber is cut at the length corresponding to a minimum width location, a broadened spectrum corresponding to the compressed pulse is obtained. To first order, only SPM contributes to the spectral broadening. Also, in DS fibers with relatively low magnitude of dispersion (i.e., D xcx9c0.2-2 ps/nm-km), the 3rd order dispersion is the dominant higher-order mechanism that affects pulse compression and, therefore, determines the spectral profile of the continuum.
Comparing ASC with SEC, SEC in the short fiber case of the present invention provides higher compression ratios than are possible with ASC and leads to super-broadened spectra in just a fraction of the soliton period. ASC requires a longer fiber because the dispersion must decrease gradually over the soliton period. Moreover, SEC is achievable in conventional, constant-dispersion DS fiber, allowing for simple implementation. While SEC does result in pedestal wings in the time domain (which means that some of the pulse energy does not contribute to the continuum generation), for soliton orders up to Nxcx9c4, the fraction of energy remaining in the wings at maximum compression is below 25 percent. In both SEC and ASC, the spectrum broadens symmetrically for an unchirped and symmetric pulse, until a higher-order dispersion or non-linear mechanism becomes significant at a sufficiently short pulse width and high peak power.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.